Prior to the present invention, thermoplastic substrates, for example, polycarbonate articles, were treated with a photocurable acrylic coating composition, as shown in Moore et al., U.S. Pat. No. 4,198,465, assigned to the same assignee as the present invention. Although the use of acrylic resins provide durable mar resistant and chemical resistant coatings, the ability of such cured polyacrylates to serve as a protective coating for a thermoplastic substrate, often did not satisfy the abrasion resistant standards required in many applications.
As shown by Clark, U.S. Pat. No. 4,027,073, an acidic dispersion of colloidal silica in a hydroxylated silsesquioxane can provide heat curable mixtures which result in valuable transparent abrasion resistant coatings on a variety of thermoplastic substrates. It was found, however, that the cure of the hydroxylated silsesquioxane binder resin often requires several hours, or even days to achieve acceptable mar resistant properties. In addition, a major drawback found with abrasion resistant coatings made in accordance with the aforementioned Clark patent is that the silsesquioxane colloidal silica coating does not adhere well to polycarbonate surfaces unless specialized surface treatments, for example primers, are used.
In the aforesaid copending application Ser. No. 269,122, valuable results were obtained by utilizing a UV curable mixture of a polyalkoxysilylacrylate, aqueous colloidal silica and polyfunctional acrylic monomer. The mixture was convertible to an adherent organopolysiloxane hardcoat in several seconds or less during radiation curing.
I have now discovered that highly abrasion resistant coatings for various thermoplastic substrates, for example, polycarbonate, exhibiting good optical quality and weatherability, also can be obtained from heat curable compositions containing colloidal silica and silyl-containing polyacrylate. The silyl-containing polyacrylate can either be, for example, a methylmethacrylate/.gamma.-methacryloxypropyltrimethoxysilane copolymer or a mixture of methylmethacryate/methacrylic acid copolymer and an epoxy silane. The aforementioned heat curable compositions can be applied onto various thermoplastic substrates and thereafter heated to effect the separation of volatiles, followed by curing of the resulting residue to produce a weather resistant coating having good optical quality and abrasion resistance.